Preheat oven to 200°C/fan 180°C/ gas 6. Whizz the flour, butter and icing sugar in a food processor until it resembles breadcrumbs. Add the egg and 2tbsp water. Whizz again to form a ball. Roll the pastry very thinly on a floured surface until it’s a little bigger than a 28cm (11in) diameter loose-bottomed tart tin. Line the tin with the pastry, letting the extra hang over the sides. Place on a baking tray and chill for 30 minutes. Place baking parchment over the pastry, cover with baking beans and bake blind for 15 minutes until pale golden. Remove the beans and paper and trim the excess pastry. Return to the oven for 10-12 minutes; set aside to cool. Reduce oven to 160°C/ fan 140°C/gas 3. Whisk the filling ingredients in a bowl until smooth, then pour into the pastry. Bake for 40-45 minutes until just set. Cool completely. Remove from the tin, sprinkle with redcurrants and dust with icing sugar.